Magnetorheological (MR) fluids include ferromagnetic particles that are suspended in a carrier fluid such as natural or synthetic hydrocarbon oil. The apparent viscosity of the MR fluid is rapidly modified when MR fluid is exposed to a magnetic field, with associated torque transmitting properties of the MR fluid increasing in conjunction with the increase in viscosity. This process is also readily reversible, thus allowing MR fluids to reversibly and almost instantaneously change from a free-flowing liquid to a semi-solid form.
In an MRF clutch in particular, controllable change in the yield stress of the MR fluid is produced by the formation of columns or lines of magnetized particles across one or more annular spaces or working gaps containing the MR fluid, with the alignment occurring in the direction of the applied magnetic field. An increase in yield stress can be used for various useful purposes, e.g., to control the fluid coupling effect between two rotating members, to change the flow rate of the MR fluid through a conduit or an orifice, etc. The change in yield stress occurs in milliseconds, and is readily reversible as noted above. Since the magnetic field can be controlled by the application of an electrical current to a field coil of an electromagnet, the yield stress of the MR fluid, and thus the applied clutch torque, can be precisely controlled.
An MRF clutch utilizes the phenomenon described above to provide a variable torque output. However, MRF clutches have a number of inherent limitations, one being a relatively low torque density in comparison to other torque-transmitting devices, e.g., hydraulically-actuated clutches. Another limitation is the potentially high electrical power consumption of any electromagnet used as a magnetic field generator. The result can be a relatively large clutch package, and/or an increased rate of power consumption, each of which typically corresponds with an elevated MR fluid temperature and corresponding fluid oxidation rate. An elevated MR fluid temperature and oxidation rate in turn can shorten the useful service life of the MR fluid, and potentially the useful service life of the MRF clutch itself.